Multifunctional Gold Nanoparticles@Imidazolium-Based Cationic Covalent Triazine Frameworks for Efficient Tandem Reactions

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Sep 2021Multifunctional Gold [email protected] Cationic Covalent Triazine Frameworks for Efficient Tandem Reactions Chang He, Qiu-Jin Wu, Min-Jie Mao, Yu-Huang Zou, Bai-Tong Liu, Yuan-Biao Huang and Rong Cao Chang He State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Qiu-Jin Wu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Min-Jie Mao State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Yu-Huang Zou State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Bai-Tong Liu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Yuan-Biao Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Fujian College, University of Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author and Rong Cao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Fujian College, University of Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000460 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Tandem catalytic reactions have attracted extensive interest because of their ability to reduce reaction steps, energy consumption, and waste. However, the construction of highly efficient tandem catalytic systems is still a significant challenge due to the problematic integration of multiple active sites in one reaction system and the incompatibility of different reaction conditions. Although metal nanoparticles (MNPs) supported on porous framework materials have shown excellent catalytic performances in various reactions, their cooperative catalysis for tandem reactions is rarely reported. In this work, ultrafine gold nanoparticles (AuNPs) stabilized by imidazolium-functionalized cationic covalent triazine frameworks (ICTFs) were employed as multifunctional catalysts for a tandem reaction completed cooperatively by Lewis acid and base sites and Au species. The multifunctional catalyst, [email protected], obtained exhibited excellent catalytic activity and selectivity in a tandem deacetalization-Knoevenagel condensation-reduction reaction under mild conditions, promoted by Lewis acid and base in ICTF and AuNPs, respectively. To the best of our knowledge, this is the first report using CTFs as multifunctional catalysts with multiple active sites to achieve catalysis of a tandem reaction cooperatively. Download figure Download PowerPoint Introduction Until now, multistep reactions are usually required to produce chemicals. From the green and sustainable synthesis viewpoint, it is extremely desirable to obtain a targeted product with a high yield efficiently and economically over a one-pot tandem catalyst system to replace multistep reactions. This is because a tandem or sequential reaction consists of two or more successive independent reactions performed in one system, thereby avoiding time-consuming separation and purification processes of intermediates to reduce energy consumption and cost significantly.1–4 These merits provide tandem reactions the opportunities to improve conversion efficiencies of many chemical transformations such as hydrogenation reactions, alkene metathesis, and Michael/Morita–Baylis–Hillman reactions.5–7 However, many chemical products such as polysubstituted arylamines still could not be achieved via tandem reactions. It is well known that arylamines containing multifunctional groups are one key class of intermediates in synthesizing dyes and drugs such as the widely used anti-inflammatory drug acetaminophen.8–10 Generally, the synthesis of such substances require multistep reactions.11,12 Therefore, developing multifunctional tandem catalyst systems with multiple active sites to synthesize such chemical compounds is highly desirable. For instance, polysubstituted arylamines containing a nitrile group, 4-amino-α-(hydroxymethyl)-benzenepropanenitrile (Figure 1b) can be synthesized by selective hydrogenation of the readily available substituted nitroarenes. Other groups can be introduced into aromatic amines through a classic Knoevenagel condensation reaction between aromatic aldehydes and nitriles over Lewis base sites. However, in such reactions, the aromatic aldehydes are easily oxidized under ambient conditions, hence, difficult to store. Therefore, to start the reaction with acetals to synthesize substituted nitroarenes via a tandem reaction of deacetalization-Knoevenagel condensation is a highly desirable strategy. As we know, the deacetalization reaction is usually catalyzed by acidic sites, while the Knoevenagel condensation reaction can be promoted by basic sites, and the hydrogenation reaction could be completed by metal nanoparticle (MNP) sites. However, designing multifunctional catalysts with high activity and selectivity to realize such tandem reaction still imposes considerable challenges such as the neutralization of the acid and base sites, the aggregation of metal NPs, and selective reduction of nitro and olefinic bonds while sustaining nitrile groups. Therefore, to synthesize the target product via tandem catalysis reaction, developing a multifunctional heterogeneous catalyst containing acidic, basic, and metal sites is a promising approach to realize enhanced catalysis cooperatively. Figure 1 | (a) Synthetic route for the multifunctional catalyst [email protected] (b) Tandem reaction involving deacetalization via Lewis acid sites of ICTF, Knoevenagel condensation via Lewis base sites of ICTF, and subsequent selective reduction via AuNPs of [email protected] ICTF, imidazolium-functionalized cationic covalent triazine framework. Download figure Download PowerPoint Recently, porous covalent triazine frameworks (CTFs) constructed through the trimerization reaction of aromatic nitriles have received much attention due to their remarkable features such as high thermal and chemical stability, large surface area, and nitrogen-rich atoms.13 Thus, multifunctional CTFs have been widely utilized in gas adsorption, separation, and catalysis.14–16 Specifically, MNPs could be stabilized by the nitrogen sites in porous CTFs to promote heterogeneous catalysis effectively.17,18 However, previous works mainly focused on investigating single-step reactions using CTFs-based catalysts, while multistep tandem reactions were rarely reported. Interestingly, compared with the neutral CTFs ( Supporting Information Figure S1),15 the imidazolium-functionalized cationic CTFs (ICTFs; Figure 1a and Supporting Information Figure S1)19–21 containing imidazolium cations and nucleophilic halogen anions could be employed as a Lewis acid and base sites for catalysis. Hence, although the ICTFs could be utilized to promote bifunctional acid-base catalysis, reports on this reaction scheme are limiting.22,23 Furthermore, the nitrogen atoms on the triazine rings of CTF show basicity and promote base-catalyzed reactions.24 Thus, we envisioned that the coexistence of Lewis acid and base sites in ICTFs might make them effective catalysts for tandem deacetalization-Knoevenagel condensation reactions. Moreover, imidazolium cations and abundant N species in porous ICTF benefit the fabrication of highly dispersed ultrafine MNPs. This is because the anionic metal-salt precursors (e.g., AuCl4−) could be absorbed into ICTF via electrostatic interaction,25–28 while the MNPs could be stabilized by the N and the in situ formed N-heterocyclic carbene (NHC) species.29–32 Therefore, in this work, we prepared highly ultrafine gold nanoparticles (AuNPs) encapsulated in ICTF ([email protected]) via a double-solvent method (DSM),33–35 followed by reduction with NaBH4. The well-designed [email protected] multifunctional catalyst could promote tandem catalysis successfully employing deacetalization-Knoevenagel condensation-reduction reaction to synthesize polysubstituted arylamines containing a nitrile group. Such a target product can be converted into acrylonitrile derivatives via a simple elimination reaction, which are essential intermediates in organic synthesis and pharmaceutical industries.36,37 The cooperation between the ICTF host and the AuNP guest makes the tandem reaction proceed smoothly. In the process, the imidazolium-based cationic frameworks in ICTF as Lewis acid sites promoted the deacetalization reaction, while the free chloride anions and triazinic N species as Lewis base sites catalyzed the Knoevenagel condensation reaction. Finally, the AuNPs act as hydrogenation sites to selectively reduce the nitro, ester groups, double carbon bonds to afford the polysubstituted arylamines containing a nitrile group. As far as we know, this is the first report using CTF as a multifunctional catalyst with multiple active sites for cooperative catalysis of a tandem reaction. Experimental Methods Preparation of ICTF The ICTF was synthesized according to our previous work on the ionothermal method.20 Typically, the monomer 1,3-Bis(4-cyanophenyl)imidazolium chloride ([BCIM]Cl) (306 mg, 1 mmol) and ZnCl2 (1.36 g, 10 mmol) were blended thoroughly in a glovebox to give a homogeneous mixture, which was then transferred into a quartz tube (1.5 × 17 cm). After evacuation and sealing, the quartz tube was heated to 400 °C, with the reaction proceeding for 40 h. After cooling to room temperature, the quartz tube was opened carefully, and the black bulk sample was ground fully and stirred in dilute HCl for 3 days to remove residual ZnCl2 entirely. Finally, the crude ICTF sample was washed three times with 50 mL water, ethanol, and tetrahydrofuran (THF), then dried in a vacuum at 70 °C overnight to obtain the pure form. Preparation of [email protected] via a DSM In a typical synthesis, a 50 mg ICTF (the pore volume of ICTF was 0.6 cm3 g−1) sample that pre-activated at 120 °C for 12 h was added to 15 mL of dried n-hexane and sonicated for 30 min. Then, 20 μL of HAuCl4 aqueous solution containing 2 mg HAuCl4·6H2O was added dropwise into the above suspension while stirring vigorously for 6 h, followed by n-hexane removal to obtain AuCl4−@ICTF product. Then the product was suspended in 10 mL methanol, and a freshly prepared 3 mL methanol solution containing 10 mg NaBH4 was added and stirred under vigorous for 5 h at 273 K, in which the AuCl4− was reduced and [email protected] was obtained by washing twice with acetone and dried in vacuum at 70 °C for further use. The Au content in [email protected] was 0.68 wt% determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Catalytic deacetalization reactions Typically, 30 mg of the dried [email protected] was dispersed in 15 mL of deionized water containing 0.5 mmol 4-nitrobenzaldehyde dimethyl acetal. Then the mixture was sonicated to be homogeneous and stirred at 60 °C in the air for 6 h. Subsequently, the catalyst was separated by centrifugation, while the solution was extracted with 3 × 15 mL ethyl acetate, and the organic phase was dried with anhydrous MgSO4, followed by solvent evaporation. The conversions were determined by gas chromatography–mass spectrometry (GC–MS). For comparison, parallel experiments were performed using 30 mg of ICTF, 30 mg of CTF-1 and 30 mg of [BCIM]Cl, independently, under the same conditions. The catalytic Knoevenagel condensation reactions Typically, 30 mg of the dried [email protected] was dispersed in 15 mL of deionized water containing 0.5 mmol ethylcyanoacetate and 0.5 mmol p-nitrobenzaldehyde, and the mixture was sonicated to be homogeneous. Then the resultant mixture was stirred at room temperature in air for 1 h. After the reaction, the catalyst was separated by centrifugation. The solution was extracted with 3 × 15 mL CH2Cl2 and dried with anhydrous MgSO4, and the solvent was evaporated to obtain the target product. The conversions and yields were monitored by GC, GC–MS, and 1H NMR spectroscopy. For comparison, 30 mg of ICTF, 30 mg of CTF-1, 30 mg of [BCIM]Cl was used independently for the reaction under the same conditions. Catalytic reduction of p-nitrophenol Typically, 30 mg of the dried [email protected] was dispersed in 30 mL of deionized water containing 0.5 mmol p-nitrophenol and 1 mmol NaBH4. The mixture was sonicated to be homogeneous. Then the resultant mixture was stirred at room temperature in air for 15 min. The reaction progress was detected at 25 °C by UV–vis spectroscopy at different intervals. After the reaction, the catalyst was separated by centrifugation, while the liquid solution was extracted with 3 × 15 mL CH2Cl2 and dried with anhydrous MgSO4, and the solvent was evaporated. The conversions and yields were determined by GC–MS. For comparison, 30 mg of ICTF, 30 mg of [email protected] and 30 mg of CTF-1 was used independently for the reaction under the same conditions. Catalytic tandem deacetalization-Knoevenagel condensation-reduction reaction Typically, 30 mg of the dried [email protected] was dispersed in 30 mL of deionized water containing 0.5 mmol 4-nitrobenzaldehyde dimethyl acetal and 0.5 mmol ethylcyanoacetate. Then the resultant mixture was stirred at 60 °C in the air for 3 h. Plenty of white flocs appeared, indicating that the deacetalization-Knoevenagel condensation reaction was completed. Subsequently, NaBH4 solution (2.5 mmol in 10 mL of water) was added, and the mixture was continually stirred for 3 h at 60 °C. The reaction mixture was then extracted with 3 × 30 mL ethyl acetate. The organic phase was dried with anhydrous MgSO4, and the solvent was evaporated. The conversions and yields were determined by GC, GC–MS, and 1H NMR spectroscopy. For comparison, 30 mg of ICTF, 30 mg of CTF-1, 30 mg of [BCIM]Cl, 30 mg of [email protected] was independently used as control catalyst for the tandem reaction under the same conditions. A recyclability test was conducted as follows: after the reaction, [email protected] was separated by centrifugation and thoroughly washed with ethyl acetate. Following drying in a vacuum, the recovered catalyst was used in the next catalytic reaction. Results and Discussion Characterization of ICTF and [email protected] As illustrated in Figure 1, the imidazolium-functionalized ICTF was successfully achieved via an ionothermal trimerization reaction of [BCIM]Cl using anhydrous ZnCl2 as a porogen and Lewis acid catalyst under vacuum for 40 h.20 For comparison, the neutral CTF-1was also prepared in a parallel experiment.13 The ICP test revealed that the Zn content in ICTF was only 0.02 wt% ( Supporting Information Table S1), suggesting that the Zn species had been almost removed. The successful synthesis of ICTF was confirmed by Fourier transform infrared (FT-IR) spectroscopy ( Supporting Information Figure S2). The peak at 2218 cm−1, initially assigned to the carbonitrile-stretching band disappeared, suggesting a complete trimerization reaction had occurred. The presence of the bands of imidazole ring stretching vibration at 1575–1630 and 1550 cm−1 indicated that the imidazolium moieties were still retained in ICTF.20 The high content of nitrogen element (8.62 wt%; Supporting Information Table S1) in ICTF was verified by the elemental analysis (EA), which was consistent with the X-ray photoelectron spectroscopy (XPS; Figure 2c). The high-resolution N 1s spectra for ICTF (Figure 2c) could be deconvoluted into four different peaks at ∼398.5, 400.2, 401.1, and 402.2 eV, which were assigned to triazinic N, imidazolium N, graphitic N, and imidazolium N+, respectively.20 Moreover, the Cl− 2p peaks were apparent at 197.3 (2p3/2) and 198.5 eV (2p1/2) in Supporting Information Figure S3. However, due to the partial decomposition of the imidazolium moieties during the synthesis of ICTF at high temperatures, part of chloride ions was inserted in situ into the graphitized frameworks. Therefore, besides the presence of chloride ions, the peaks at 200.3 (2p1/2) and 201.9 (2p3/2) eV corresponding to C–Cl bond were also observed for ICTF. According to the XPS results, the ratio of free chloride ions was calculated as ∼70%. Consequently, based on the EA and XPS results, the content of the imidazolium moieties was calculated as 1.06 mmol g−1 in ICTF. Figure 2 | XPS C 1s spectra of (a) ICTF and (b) [email protected] XPS N 1s spectra of (c) ICTF and (d) [email protected] XPS, X-ray photoelectron spectroscopy; ICTF, imidazolium-functionalized cationic covalent triazine framework. Download figure Download PowerPoint The above results confirmed the existence of the imidazolium group in ICTF, suggesting that it contained Lewis acid and base sites. We investigated its Lewis acid and base sites by performing NH3 and CO2 temperature-programmed desorption (TPD) spectrometry, respectively. As shown in the NH3-TPD profiles in Figure 3a, compared with that of the neutral CTF-1, ICTF showed a distinct peak in the range of 325–375 K, which could be assigned to NH3 adsorption on the imidazolium cations,38,39 validating its Lewis acidity. In contrast, as shown in Figure 3b, a weak desorption peak at 550 K was observed in the CO2-TPD for CTF-1, ascribed to the interaction between CO2 and the triazine group in CTF-1.24 Compared with the neutral CTF-1, a broad visible peak at ∼550 K was observed in the CO2-TPD profile for ICTF, attributable to the CO2 adsorption on the free chloride anion and triazinic N,40,41 indicating its medium Lewis basicity. These results proved the coexistence of Lewis acid and base sites in ICTF, suggesting that it might catalyze tandem acid–base reactions. Figure 3 | (a) NH3-TPD and (b) CO2-TPD profiles of ICTF and CTF-1. (c) PXRD patterns of ICTF, [email protected], and [email protected] after catalysis. (d) N2 sorption isotherms of ICTF, [email protected], and [email protected] after catalysis at 77 K. TPD, temperature-programmed desorption; ICTF, imidazolium-functionalized cationic covalent triazine framework; PXRD, powder X-ray diffraction. Download figure Download PowerPoint The powder X-ray diffraction (PXRD) patterns of ICTF (Figure 3c) display only a broad peak at 2θ = 25° assigned to (002) plane reflection,20 indicating that ICTF has a similar amorphous structure as that of most CTFs. The N2 adsorption measurement demonstrated that ICTF has a higher Brunauer–Emmett–Teller (BET) surface area of 660 m2 g−1 in comparison with that of CTF (Figure 3d and Supporting Information Table S2), and the pore size distribution further confirmed their porous feature ( Supporting Information Figures S4 and S5). Such a porous structure for ICTF is beneficial for encapsulating tiny metal NPs. As demonstrated in Supporting Information Figure S6, ICTF has hydrophobic external surfaces, but pores are hydrophilic because most chloride ions fill ICTF pores. Thus, to embed AuNPs in the pores of ICTF to avoid their aggregation, a DSM was adopted to introduce the AuCl4− ions into the ICTF.33 Using this method, the metal precursor HAuCl4 aqueous solution with a volume less than that of the ICTF pore volume could be absorbed in the ICTF pores based on capillary force and hydrophilic interaction. Moreover, the electrostatic interaction between AuCl4− ions and the positive imidazolium cations of ICTF enables the anion salt to disperse highly in the pores, and thus, facilitate the subsequent formation of tiny AuNPs.25 After the addition of a highly concentrated NaBH4 solution, AuCl4− was reduced to tiny AuNPs. No diffraction peak for AuNPs was observed in the PXRD of [email protected] (Figure 3c), suggesting that small AuNPs might have been generated.33 N2 adsorption measurements revealed that [email protected] showed a decreased BET surface area (541 m2 g−1) and pore volume (0.16 cm3 g−1), compared with those of ICTF ( Supporting Information Table S2) indicating that AuNPs occupied portions of the ICTF pores. Nevertheless, [email protected] still exhibited a high N2 adsorption uptake (Figure 3d), which did not affect the diffusion of the reactants and products. The transmission electron microscopy (TEM) (Figure 4a) and high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure 4c) further revealed that highly dispersed AuNPs with an average size of 1.38 ± 0.3 nm (Figure 4d) were stabilized by ICTF. The TEM element-mapping images showed that the gold, with nitrogen and chloride elements, was homogeneously distributed in the ICTF (Figure 4e–4f). Moreover, the high-resolution TEM (HRTEM) image (Figure 4b) showed that the spacing of the lattice fringes is 0.235 nm, which is consistent with the (111) plane of face-centered cubic (fcc) Au0. The morphology of AuNPs supported on [email protected] was also investigated by TEM and HAADF-STEM. Compared with the tinny AuNPs in [email protected], the [email protected] showed larger AuNPs with a mean size of 4.7 ± 0.8 nm ( Supporting Information Figure S7), suggesting the significance of the imidazolium-based cationic frameworks in the stabilization of AuNPs. Figure 4 | (a) TEM image of [email protected] (b) HRTEM image of [email protected] The scale bar is 5 nm. (c) HAADF-STEM image of the ultrafine AuNPs in [email protected] and (d) corresponding size distribution of AuNPs. (e) HAADF-STEM image and (f) corresponding EDS mapping of Au, C, N, and Cl elements in the selected area of [email protected] TEM, transmission electron microscopy; ICTF, imidazolium-functionalized cationic covalent triazine framework; HRTEM, high-resolution TEM; HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy. Download figure Download PowerPoint The formation of such a small size of AuNPs in [email protected] might be ascribed for the following reasons. The DSM guaranteed that most of the metal precursors of AuCl4− entered the ICTF pores, which could limit the growth of AuNPs. Furthermore, the anionic AuCl4− has an electrostatic effect with the positive imidazolium cation in ICTF, which inhibited the aggregation of the AuCl4−. Moreover, the imidazolium cation groups in ICTF were transformed in situ to NHC moieties, able to coordinate and the AuNPs. the reduction process, NaBH4 could partial of the imidazolium moieties in ICTF and make them transform into which can as to coordinate Au species and to their aggregation to a bulk the nitrogen atoms in ICTF are also beneficial for The interaction between the Au species and ICTF was confirmed by the XPS As shown in Figure the C 1s of ICTF could be deconvoluted into three peaks at and eV, which were assigned to and carbon Compared with ICTF, the carbon of species in [email protected] 0.5 eV to energy due to the more nucleophilic feature (Figure consistent with the previous report of liquid to The Au and peaks of the XPS for [email protected] at and eV ( Supporting Information Figure indicating that Au species was in the reduced In contrast, were in the C 1s between CTF-1 and [email protected] ( Supporting Information Figure Compared with the N 1s XPS spectra of ICTF (Figure the of the four different N 1s peaks at and eV for [email protected] were observed (Figure suggesting the existence of an interaction between the nitrogen atoms of ICTF and the Interestingly, a similar was also in [email protected] ( Supporting Information Figure attributable to the abundant triazine moieties in the CTF-1. Catalytic performances The coexistence of Lewis acid and base sites in ICTF and ultrafine AuNPs could [email protected] as a multifunctional catalyst to cooperatively promote tandem reaction of acid–base catalysis and hydrogenation Therefore, we the catalytic of [email protected] for the sites catalyzed multistep tandem deacetalization-Knoevenagel condensation-reduction reaction. We achieved this tandem reaction via an of the catalytic performances of [email protected] for three reactions, Knoevenagel and we the deacetalization reaction of 4-nitrobenzaldehyde dimethyl acetal to (Figure As ICTF and [email protected] Lewis acid sites can efficiently promote the deacetalization reaction, and high yields of were The successful conversion of 4-nitrobenzaldehyde dimethyl acetal was to the Lewis acid sites of the imidazolium cations in ICTF and [email protected], as verified by the NH3-TPD of ICTF (Figure The monomer [BCIM]Cl also showed high indicating that the imidazolium cations a key in deacetalization reaction. In contrast, the only of over the neutral CTF-1 or catalyst was detected due to a of acid sites in CTF-1 (Figure These results indicated that [email protected] still imidazolium ions after AuNPs, which could be utilized as Lewis acid sites for deacetalization reaction. Figure 5 | of the deacetalization reaction catalyzed by [email protected], ICTF and its monomer [BCIM]Cl, and CTF-1. the yields of ICTF, imidazolium-functionalized cationic covalent triazine framework; [BCIM]Cl, 1,3-Bis(4-cyanophenyl)imidazolium Download figure Download PowerPoint to the presence of Lewis base sites in ICTF, the base-catalyzed Knoevenagel condensation reaction between ethylcyanoacetate and was investigated (Figure The ICTF and [email protected] showed high catalytic activity and completed the Knoevenagel condensation reaction in only 1 h. the monomer [BCIM]Cl exhibited a yield than the corresponding heterogeneous ICTF, suggesting that the active sites in the porous framework could be to to facilitate the In contrast, the neutral CTF-1 with weak showed activity for this base-catalyzed reaction, compared with [email protected] and ICTF. the catalyst, the Knoevenagel condensation reaction did not These results confirmed that the free chloride ions as Lewis base sites in [email protected] a key in efficient catalysis of the Knoevenagel condensation reaction. Figure 6 | of the Knoevenagel condensation reaction catalyzed by [email protected], its monomer [BCIM]Cl, and CTF-1. the yields of ICTF, imidazolium-functionalized cationic covalent

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